It is generally advantageous to minimize emissions such as nitrogen oxides (NOx), carbon monoxide, and unburned hydrocarbons of combustion gases created in a combustor of a gas turbine engine. Axial staging combustion is one approach for reducing such emissions. Axially staged combustion generally includes injecting a secondary fuel and air mixture from one or more radially oriented fuel injectors into a flow of combustion gases at a location that is downstream from a primary combustion zone. However, even with axial staging, NOx is produced in higher amounts at higher flame temperatures.
NOx emissions can be reduced by lowering the flame temperature and/or lowering the residence time of the combustion gases in high temperature zones. In contrast, as compared with NOx emissions, a longer residence time and higher temperature favors low carbon monoxide emissions. In order to balance NOx and CO emissions and to protect combustion hardware, traditional axially staged combustion systems require a large combustion volume and as such, a high volume of cooling air which may affect overall gas turbine efficiency.